(a) Field of the Invention
The present invention relates to a switch driving circuit and a driving method thereof, and particularly relates to a switch driving circuit that can prevent a malfunction due to a negative voltage generated therein and a driving method thereof.
(b) Description of the Related Art
FIG. 1 schematically shows a conventional switch driving circuit. Referring to FIG. 1, a switch driving circuit 10 is connected to both ends of a bootstrap capacitor C1 and is driven by a voltage charged therein. A bootstrap diode Db is connected between a power source Vcc and the bootstrap capacitor C1, and is turned on during a period during which a source voltage Vs is lower than a voltage of the power source Vcc so that the bootstrap capacitor C1 is charged. Hereinafter, a voltage at a first end of the bootstrap capacitor C1 is referred to as a bootstrap voltage Vb, and a voltage at a second end thereof is referred to as a source voltage Vs.
A first side of an inductor L is connected to a power switch Q, and a second end thereof is connected to a first side of a capacitor C2. A second end of the capacitor C2 is connected to a ground terminal. A diode D is connected between the power switch Q and ground.
A parasitic diode Dp shown in FIG. 1 is formed between a terminal of a bootstrap voltage Vb of the switch driving circuit 10 and ground, and a parasitic inductor Lp, typically generated between wires, is generated in wires between the power switch Q and the diode D. The power switch Q is turned on/off by the switch driving circuit 10.
However, when the power switch Q is turned off, a freewheeling current i flows in a direction (the arrow direction of FIG. 1) in which a current flowing to the inductor L through the power switch Q passes through the diode D and the inductor L. Then, the source voltage Vs becomes lower than a ground voltage. When the source voltage Vs is decreased, the bootstrap voltage Vb is also decreased along with the decrease of the source voltage Vs. In this case, when the parasitic inductor Lp exists inside a circuit wire, a voltage drop occurs at both ends of the parasitic inductor Lp due to the freewheeling current. Due to the voltage drop, the source voltage Vs is further decreased. The voltage drop that occurs at both ends of the parasitic inductor Lp is a value obtained by multiplying inductance of the parasitic inductor Lp by (di/dt). Then, the bootstrap voltage Vb becomes lower than the ground voltage so that the parasitic diode Dp of the switch driving circuit 10 is turned on. Hereinafter, this is referred to as an abnormal state. Due to the abnormal state, much more current flow to the bootstrap capacitor C1 and the bootstrap capacitor C1 is an overcharge so that element damage occurs due to an over-voltage.
Moreover, malfunction of the switch driving circuit 10 due to the turn-on of the parasitic diode Dp causes the latch-up of the power switch Q so that the power switch Q is continuously turned on regardless a switch driving control signal. Then, the entire power control system including the switch driving circuit 10 is damaged.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.